Abstract Introduction Catheter ablation (CA) has proven to be effective therapy for ventricular tachycardia (VT) in patients with structural heart disease. However, activation mapping during VT is often limited in case of non-inducible, non-sustained or hemodynamically tolerable. It has been reported that a substrate-based approach for identifying conduction block and slow-conducting sites during sinus rhythm and ventricular pacing is promising strategies for CA. However, detailed analysis on the effectiveness of substrate mapping with multidirectional pacing to identify critical isthmus in unmappable VT have been lacking. Purpose The purpose of the present study was to evaluate the effectiveness of substrate mapping with multidirectional pacing in VT ablation. Methods From April 2022 to July 2023, 7 consecutive patients (4 men, age 62±9years) who underwent CA of VTs at our institute were enrolled. Isochronal late activation mapping (ILAM) was performed during sinus rhythm, right ventricular (RV) and left ventricular (LV) pacing. Cardiovascular magnetic resonance were performed in 3 of 7 cases to assess the distribution of myocardial late gadolinium enhancement. For catheter ablation, 2Fr electrode catheter was inserted into the anterior interventricular vein (AIV) at the distal site of the coronary sinus. LV pacing was performed using an electrode catheter placed in the AIV. ILAM were constructed by annotating the last deflection of each electrogram. Results In the 7 patients, 5 had ischemic cardiomyopathy and 2 had non-ischemic cardiomyopathy (Figure 1). During sinus rhythm, a deceleration zone (DZ) was identified in only 1 of 7 cases, consistent with linear conduction block. RV pacing demonstrated DZ in 3 of 7 cases. LV pacing revealed DZ in 6 of 7 cases, all of which had conduction blocks. U-shaped conduction blocks, which was not detectable in activation mapping during sinus rhythm, was newly identified in 29% of RV pacing, 71% of LV pacing. Combined RV and LV pacing unmasked the U-shaped conduction block in 86% of the cases. Only in case 7, activation mapping of VT with a cycle length of 470ms could be performed, and VT isthmus activation was observed in the area surrounded by the U-shaped conduction block (Figure 2). In other cases, the clinical VTs were unmappable because of the hemodynamic instability with tachycardia cycle length of approximately 300 milliseconds. Eventually, VTs became non-inducible in 3 of 5 cases in which induction was performed after the CA. During the follow-up period of 7±5 months, 6 of 7 cases remained free from any VT episodes. One patient showed small amount of pericardial effusion after CA. However, there were no life-threatening complications in any patients. Conclusion Substrate mapping using several different wavefront vectors in unstable VTs unveiled conduction block zones that might be potential ablation target sites for VT.Figure 1Figure 2

Manifestation of conduction block zones by substrate mapping evaluated under multidirectional pacing during ventricular tachycardia ablation